The present invention relates to x-ray lasers employing hydrogenic lasing ions of nuclear charge Z, for n=2 to 4 or 6 excitation, and radiating ions of nuclear charge Z/2 for decreasing the n=2 population density of the hydrogenic lasing ions of nuclear charge Z.
A goal of x-ray lasers is to provide the most advantageous levels of population inversion, gain, and size. Currently, radiation trapping creates one of the major limitations on population inversion, gain, and size in x-ray lasers.
It is likely that reaching a short-wavelength plateau of less than 50 angstroms in laboratory x-ray laser development will depend on .DELTA.n=1 transitions. Currently, it appears that the .DELTA.n=0 transitions cannot reach this plateau. Some of the attempts for achieving this plateau are discussed in my paper published in Physical Review A, Volume 38, No. 10, 5426 (1988).
Matthews et al, J. Opt. Soc. Am. B 4,575 (1987), showed that the successful 3p.fwdarw.3s neonlike ion transition does not readily extrapolate to the less than 50 angstrom plateau. MacGowan et al, Phys. Rev. Lett. 59, 2157 (1987), showed that the inherent multiplicity of the n=4 to 4 nickel-like transitions limit the achievable gain. Thus, for .DELTA.n=1 transitions, hydrogenic ions are attractive candidates for reaching the less than 50 angstrom short-wavelength plateau.
There has been particular success in lasers with the C.sup.5+ ion at 182 angstroms on the n=3 to 2 Balmer- .alpha. transition. The lasing wavelength for this transition extrapolates as Z.sup.-2 (Z is nuclear charge), e.g., extrapolates to 45 angstroms for Mg.sup.11+. However, there is a problem with this transition because the size becomes micrometer in scale due to radiative trapping on the 2p-1s Lyman-.alpha. resonance transition.
Of the known pumping methods, electron-collisional recombination has proven to be an effective pumping method for producing population inversions leading to amplified spontaneous emission in the xuv spectral region, with the gain scaling hydrogenically as approximately Z.sup.7.5. Pumping is achieved in a high-density plasma consisting of totally stripped ions of the element of laser interest in which the electrons are suddenly cooled, leading to rapid collisional recombination (scaling as T.sub.e.sup.-2) and cascading. When the ion temperature T.sub.i is also low, an additional enhancement of the overall gain (scaling as T.sub.i.sup.-1/2) is obtained through reduced Doppler line broadening. A further advantage of the present .DELTA.n=1 recombination-pumped devices is that they operate at a lower electron density than the electron-collision-pumped .DELTA.n=1 devices for a similar wavelength. Still further, the .DELTA.n=1 recombination-pumped devices have the added advantage of lower refraction losses through the amplifying line plasma.
In spite of these obvious advantages, the measured gain coefficients of these devices are capped at about 3-6 cm.sup.-1. This limitation can be associated with a relative population inversion factor 1-N.sub.2 g.sub.3 /N.sub.3 g.sub.2 which just marginally exceeds zero, due to collisional mixing and radiative trapping at the high densities required for such gain (the upper- and lower-state densities are designated respectively as N.sub.3 and N.sub.2, and the statistical weights as g.sub.n =2n.sup.2 (n=2,3) for the Balmer- .alpha. transition). In other words, the population inversion decreases due to collisional mixing and radiative trapping as the density is increased for higher gain. Thus, N.sub.3 .apprxeq.N.sub.2 (g.sub.3 /g.sub.2).
Therefore, in order to increase the population inversion and thereby the gain to saturation, as well as to improve the overall efficiency, and to increase the plasma size, it is important to decrease the lower state population density N.sub.2.
To further improve efficiency, it is necessary to better utilize the driving laser energy. Presently, approximately 90% of driving laser energy is lost in laser heated thin targets, probably to kinetic energy of expansion and transmission.